In this type of the pneumatic radial tire, it is common that at least two belt layers, each a composite of steel cords arranged at a given inclination angle with respect to an equatorial direction of the tire and coated with rubber, are superimposed on a carcass to form a structure providing steering performance, wear resistance and the like in accordance with vehicle performance.
Recently, the running speed of the vehicle has increased with the advance in the improvement of expressway networks as well as higher output and performance of the vehicle, and it is strongly demanded to improve high-speed durability of a tire accompanied therewith.
For this end, there have been proposed various attempts for improving the high-speed durability of the tire. That is, the pushing-out of tread or shoulder in the tire through centrifugal force acting to the tread portion of the tire during high-speed running, and the formation of a standing wave are serious factors causing trouble for the tire. In order to suppress such a trouble, it is known to add belt reinforcing layer such as a so-called cap, layer or the like to an outside of the belt layer in a radial direction of the tire.
However, it is difficult to completely make up for a shortage of rigidity by a simple addition of the belt reinforcing layer. Consequently, the size growth of the tire in the radial direction during the high-speed running can not be sufficiently suppressed. For this end, it is attempted to improve the rigidity by increasing the number of the belt reinforcing layers. However, the increase of the belt reinforcing layer number increases the rubber thickness to bring about heat build-up or the increase of tire weight, so that the satisfactory high-speed durability can not be obtained.
Furthermore, when using the plural belt reinforcing layers, the dimensional stability of the tire from heat build-up is degraded. Also, the heat build-up becomes large, so that there is caused a problem of lowering the steering stability during running for a long time.
On the other hand, it has been attempted to improve rigidity by increasing an end count of cords embedded in rubber for the belt reinforcing layer. However, as the end count increases, a ratio of cords occupied per unit width in the radial section of the tire or a cord occupying ratio becomes large and a ratio of occupied rubber becomes relatively small. Hence the adhesion property of the cord lowers to induce separation failure or the like and hence there are caused a premature failure of tire and the like.
As compared with these attempts, it is effective to enhance a modulus of elasticity of the cord constituting the belt reinforcing layer. Concretely, it is known to use a high modulus material such as aromatic polyamide (aramid) fiber, polyethylene terephthalate (PET) fiber or the like. As another means, JP-B-59-1601 proposes a method wherein polyamide fibers, polyester fibers or rayon fibers, particularly polyamide fibers are used to form a cord of single twisting structure having a twisting number of 10-40 turns/10 cm and then the resulting cords are used at an end count of 60-300 cords/10 cm in the belt reinforcing layer.
However, when cords of aramid fiber or the like having a low elongation are used as a cord for the belt reinforcing layer, there may be caused tire deformation or cord breakage due to the expansion of the tire during the tire build-up. On the other hand, when the tire expansion during the tire build-up is made small in order to prevent such disadvantages, since the thermal shrinkage of the aramid fiber cord is substantially zero, the cords undesirably move in a zigzag direction during the vulcanization of the tire.
When using the polyester fibers such as PET fiber or the like, since the modulus of elasticity is higher than that of the polyamide fiber, the size growth of the tire is suppressed. If they are used under high-speed running, the heat-resistant adhesion property to other parts of the tire is poor, from which separation failure is easily produced, which results in trouble with the tire.
Moreover, in case of using the single twisted cord is used, since there is a residual torque in the cord itself, the operability is poor in the weaving step and the dipping step with an adhesive, which is easy to produce troubles in tire production. Furthermore, the high-speed durability of the tire cannot be said to be satisfactory even if the twisting number and end count of the cord are defined to given values. Therefore, drastic measures are required.